1. Field of the Invention
This invention relates to electric discharge tube apparatus of the kind in which a high frequency voltage of gradually increasing amplitude is applied to a gas discharge tube whereby the luminous discharge is made to extend gradually along the tube from its excited end till it fills the whole length of the tube thereby producing a "writing" effect.
Such discharge is initiated as soon as the intensity of the electric field between the excited electrode and its background reaches a level sufficient to ionise gas in the vicinity of the electrode. The electric field between the gas thus ionised and its background ionises adjoining gas, and as the voltage, and therefore the intensity of the electric field, continues to rise, a luminous discharge column builds up along the tube with a flowing action. The discharge column acts as an extension of the electrode and forms a capacitance with its background, which provides the return path for the discharge current.
If, however, the tube is bent to form the letters of a word or a design, its configuration inevitbly includes sections such as double-backs, loops and the like, where different points on the tube, at some distance from each other along its length, come into close proximity. The discharge does not progress evenly along such sections with a flowing action but rather with a flash-on action, causing the whole section or parts thereof to light up suddenly, thus spoiling the desired writing effect. This erratic behaviour is due to the wide and ill-defined distribution of the electric field in the air space between the discharge columm and its background, whereby it encloses and ionises the gas in the said sections of the tube.
2. Description of the Prior Art
To cure the trouble it is necessary, therefore, to confine the electric field within proper bounds. Means for carrying this into effect are described in United Kingdom Patent No. 1,063,262, and consist of an earthed wire supported and spaced a few millimeters from the surface of the tube along its length. The electric field is thus largely confined to the space within the capacitance formed by the discharge column and the wire. The wire is spaced from the tube because, if it were applied direct to the surface of the tube, (a) the voltage gradient between the discharge column and the wire could be sufficient to induce considerble corona and formation of ozone, or even to puncture the glass wall of the tube, and (b) the discharge current could become unduly large owing to the relatively large capacitance between the discharge column and the earthed wire, resulting in a steep current density gradient along the discharge column, and therefore a marked change in light intensity along its length.
This spaced wire system nevertheless has several shortcomings. Securing the wire to the foamed plastic strip usually employed as the spacing medium, followed by application of the strip along the tube is laborious and time-consuming. Due to its thickness the strip can be unsightly, and is different to accomodate in the narrow gap between the parallel limbs of double-backs in the configuration of the tubing. Also the tube has to be handled with care to avoid damage or dislodgment of the vulnerable strip. Moreover, despite the said spacing, corona and the formation of ozone cannot always be entirely eliminated if the length of tubing operated exceeds 20 feet.